The present invention relates to a decoder that converts a digital television signal into an output video signal, and more specifically to a method and apparatus for synchronizing format changes in a digital television signal with corresponding format changes for an on-screen display.
In the United States a standard has been adopted for terrestrially broadcast digitally encoded high definition television signals (HDTV). A guide to the use of this standard entitled, xe2x80x9cGuide to the Use of the ATSC Digital Television Standardxe2x80x9d, Doc. A/54 (1995), is provided by the Advanced Television Systems Committee (ATSC), and is hereby incorporated by reference. A portion of this standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). The MPEG-2 standard is described in an International Standard (IS) publication entitled, xe2x80x9cInformation Technologyxe2x80x94Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626xe2x80x9d, ISO/IEC 13818-2, IS, November 1994 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard. The MPEG-2 standard is also used for digital cable transmissions and digital satellite transmissions.
This standard defines a complex syntax that contains a mixture of data and control information. Some of this control information is used to enable signals having several different formats (resolutions) to be covered by the standard. These formats define images having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field, and differing numbers of frames or fields per second. These formats are typically referenced by the number of horizontal lines in the image and whether each image frame is formed from two fields, each containing alternate lines of the frame (interlaced) or from a single image field containing all of the lines of the frame (progressive). Listed from highest resolution to lowest resolution, the television signal formats defined by the ATSC standard, for example, are referenced by the designations, 1080I, 720P, 480P and 480I. In these designations, the number refers to the number of horizontal lines in the image and the letter defines the resulting image as being interlaced (I) or progressive (P).
Because these formats define different numbers of lines per image field or frame, they inherently change the way in which the display device operates. Some display devices, generically referred to as multisync monitors, can display more than one format. Two types of multisync monitors are Native Mode Monitors and Hybrid Mode. Native mode display devices support all of the ATSC formats and hybrid mode displays support all formats except for 1080I. A broadcaster may change the format of the digital data that it sends, for example, from 480I to 720P. If the viewer uses a set-top box such as the TU-DST51 DTV Decoder Set-Top Box manufactured by Panasonic with a multisync monitor, the viewer may also change the display format independently of the format of the received signal. Thus, the viewer who receives the 720P program may choose to display it as a 480P video sequence.
Thus, a digital television decoder, such as the above-referenced set-top box, may decode the mixture of data and control information in the ATSC or MPEG bit stream and provide a video signal for displaying a video image in a format specified in the bit-stream or requested by the viewer. An on-screen display (OSD) processor of a decoder may be used to provide an OSD in combination with the video image. The OSD is used, for example to display control menus and channel guides. Because the format of the digital signal (480I, 480P, 720I, etc.) may change as necessary and may be indicated as part of the data and control information, the format of the on-screen display needs to change so that, at any given time, it is compatible with the current format of the display device. Due to the number of ways that the display format can be changed, situations may exist when the OSD has been programmed to provide a display in one format but the display device has been converted to another format. In this instance, the on-screen display may be distorted or unreadable.
To prevent the user from viewing an incorrectly formatted OSD, it is desirable to inhibit the on-screen display during this period of transition among display formats and re-enable the OSD once it is synchronized with the current format of the digital television signal.
The present invention is embodied in apparatus and a method for synchronizing a digital television signal and an on-screen display. The apparatus includes a video decoder which receives a digital television signal, determines the display format to be used, and generates an output signal based on the format. The apparatus also includes a user interface coupled to the video decoder and to an on-screen display processor to receive the output signal and generate data to be displayed as the on-screen display. The on-screen display processor is coupled to the video decoder and the user interface to receive the output signal and the OSD data. When the formats of the output signal and the OSD data match, the format of the digital television signal and the on-screen display are synchronized and the on-screen display is enabled.
According to another aspect of the invention, the format is part of the digital television signal.
According to a further aspect of the invention, the format is determined by the user.
The method for synchronizing an on screen display with a digital television signal includes determining a current display format for the digital television signal; comparing the current format with a previous format; and selectively displaying an image based on the comparison.
According to one aspect of the invention, the method assigns an identifier to the format; provides the identifier to a user interface and the on-screen display; and provides to the on-screen display a further identifier from the user interface based on the identifier provided to the user interface.
According to another aspect of the invention, the on-screen display is enabled if the identifier and the further identifier match.
According to yet another aspect of the invention, the on-screen display is disabled if the identifier and the further identifier that are most recently received do not match.